feat(pylibmc): wrap additional commands

Wraps the common memcached add command.

Testing & risks: Add is very similar to the existing set and set_multi wrapped functions.

Checklist

[x] Change(s) are motivated and described in the PR description
[x] Testing strategy is described if automated tests are not included in the PR
[x] Risks are described (performance impact, potential for breakage, maintainability)
[x] Change is maintainable (easy to change, telemetry, documentation)
[x] Library release note guidelines are followed or label changelog/no-changelog is set
[x] Documentation is included (in-code, generated user docs, public corp docs)
[x] Backport labels are set (if applicable)
[x] If this PR changes the public interface, I've notified @DataDog/apm-tees.

[ ] Title is accurate
[ ] All changes are related to the pull request's stated goal
[ ] Description motivates each change
[ ] Avoids breaking API changes
[ ] Testing strategy adequately addresses listed risks
[ ] Change is maintainable (easy to change, telemetry, documentation)
[ ] Release note makes sense to a user of the library
[ ] Author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
[ ] Backport labels are set in a manner that is consistent with the release branch maintenance policy